Controlled preparation of cerium oxide loaded slag-based geopolymer microspheres (CeO2@SGMs) for the adsorptive removal and solidification of F- from acidic waste-water

Synthesis of BiOX-Red Mud/Granulated Blast Furnace Slag Geopolymer Microspheres for Photocatalytic Degradation of Formaldehyde

Release of formaldehyde gas indoors is a serious threat to human health. The traditional adsorption method is not stable enough for formaldehyde removal. Photocatalytic degradation of formaldehyde is effective and rapid, but photocatalysts are generally expensive and not easy to recycle. In this paper, geopolymer microspheres were applied as matrix materials for photocatalysts loading to degrade formaldehyde. Geopolymer microspheres were prepared from red mud and granulated blast furnace slag as raw materials by alkali activation. When the red mud doping was 50%, the concentration of NaOH solution was 6 mol/L, and the additive amount was 30 mL, the prepared geopolymer microspheres possessed good morphological characteristics and a large specific surface area of 38.80 m2/g. With the loading of BiOX (X = Cl, Br, I) photocatalysts on the surface of geopolymer microspheres, 85.71% of formaldehyde gas were adsorbed within 60 min. The formaldehyde degradation rate of the geopolymer microspheres loaded with BiOI reached 87.46% within 180 min, which was 23.07% higher than that of the microspheres loaded with BiOBr, and 50.50% higher than that of the microspheres loaded with BiOCl. While ensuring the efficient degradation of formaldehyde, the BiOX (X = Cl, Br, I)-loaded geopolymer microspheres are easy to recycle and can save space. This work not only promotes the resource utilization of red mud and granulated blast furnace slag, but also provides a new idea on the formation of catalysts in the process of photocatalytic degradation of formaldehyde.

A novel amine functionalized porous geopolymer spheres from municipal solid waste incineration fly ash for CO2 capture

Municipal solid waste (MSW) incineration fly ash (FA) is classified as hazardous waste, and strategies for recycling FA have attracted attention. In this study, the porous geopolymer spheres (PGS) were prepared from FA by the foaming-suspension-solidification method, and then the PGS were functionalized with tetraethylenepentamine (TEPA) to capture CO2. The results showed that washing pretreatment and the addition of H2O2 foaming agent enhanced the pore volume and specific surface area of PGS. The CO2 adsorption capacity of amine-functionalized PGS exhibited a trend of increasing and then decreasing in the range of 35-80 °C. The maximum adsorption capacity of TEPA-WPGS3 was 2.55 mmol/g at 65 °C higher than expected for the average of TEPA and PGS. This was because PGS improved the dispersion of TEPA, thus exposing more active sites of TEPA and making it more likely to interact with CO2. The adsorption efficiency of amine-functionalized PGS decreased by only 2.4% after 10 cycles, indicating that it has excellent regeneration performance. In addition, amine-functionalized PGS, which showed excellent CO2 adsorption capacity, had a significant ability to selectively adsorb CO2 and the adsorption capacity of the rapid stage accounted for approximately 80% of the saturated adsorption capacity. This study shows that FA-derived geopolymers have excellent CO2 adsorption properties and provides a new method for the resource utilization of FA.

Efficient preparation of red mud-based geopolymer microspheres (RM@GMs) and adsorption of fluoride ions in wastewater

In this paper, red mud-based geopolymer microspheres (RM@GMs: 75-150 µm) was prepared by dispersion-suspension-solidification method to remove fluoride ions (F^-). It was found that RM@GMs still had good mechanical properties and better F^- removal effect at RM content reached 80 % of the total solid mass. The batch adsorption experiment results showed that the F^- concentration (< 1.5 mg/L) reached the drinking water standard in 45 min at pH = 2 and RM@GMs dosage was 1 g/L. RM@GMs showed maximum adsorption capacity of 76.57 mg/g for F^-, and the adsorption kinetics and isotherm fitted the pseudo-second-order kinetic and Langmuir isotherm model, respectively. RM@GMs exhibited excellent dynamic separation effect at the flow rate of 4 mL/min and column height of 1 cm. In addition, RM@GMs had good selectivity for F^- in the competitive adsorption experiments and followed an order of: PO₄^3- > > SO₄^2- ≈ NO₃^- ≈ Cl^-. In real seawater, natural surface water and tap water, RM@GMs still had excellent F^- removal effect. The adsorption mechanism revealed that RM@GMs removed F^- mainly through the synergistic effect of adsorption and ion exchange. Therefore, this paper provides the potential value for the large-scale utilization of RM in the application of F^--containing wastewater.

A critical review on adsorption and recovery of fluoride from wastewater by metal-based adsorbents

Rapid industrialization is deteriorating water quality, and fluoride pollution in water is one of the most serious environmental pollution problems. Adsorption technology is an efficient and selective process for removing fluoride from aqueous solutions using adsorbents. Metal-based adsorbents synergize the advantages of fast adsorption, high adsorption capacity, and excellent selectivity to effectively remove fluoride from water bodies, promising to satisfy environmental sustainability requirements. This paper reviews the metal-based adsorbents: iron-based, aluminum-based, lanthanum-based, cerium-based, titanium-based, zirconium-based, and multi-metal composite adsorbents, primarily focusing on the adsorption conditions and fluoride removal capacities and discusses prospects and challenges in the synthesis and application of metal-based adsorbents. This paper aims to stimulate new thinking and innovation in developing the next generation of sustainable adsorbents.

Environmental application of engineering magnesite slag for phosphate adsorption from wastewater

Herein, magnesite slags (MS), which remain after sulfuric acid extraction from light burnt magnesite in the magnesite industry, were used as phosphate adsorbents in wastewater. The MS were calcined under 700 °C to enhance phosphate adsorption. The calcined magnesite slags (CMS) were characterized by nitrogen adsorption-desorption isotherm, X-ray diffraction, and scanning electron microscopy. A series of batch adsorption experiments were carried out to test the phosphate adsorption capacity of CMS. The results showed that the calcific treatment promoted the conversion from Mg, Ca, Fe, etc. compound to metal oxide of the MS. The generated metal oxide particles resulted in 237.4 mg/g increase in the phosphate adsorption capacity. The phosphate adsorption isotherm of CMS fitted the Langmuir model better, and the maximum adsorption capacity of CMS was 526 mg/g. The adsorption kinetics of phosphate on CMS can be described by the pseudo-second-order model. The phosphate removal efficiency was greater than 98% in 300 mg/L phosphate solution. Mechanism investigation results indicated that phosphate was adsorbed by CMS through MgO protonation, electrostatic attraction, Mg-P complexation, and ligand exchange. The results obtained in this work demonstrate that the CMS is a potential effective adsorbent for removal and reutilization phosphate from P-contaminated water, due to it can be employed as a fertilizer after phosphate adsorption.

Application of Geopolymer in Stabilization/Solidification of Hazardous Pollutants: A Review

Geopolymers, as a kind of inorganic polymer, possess excellent properties and have been broadly studied for the stabilization/solidification (S/S) of hazardous pollutants. Even though many reviews about geopolymers have been published, the summary of geopolymer-based S/S for various contaminants has not been well conducted. Therefore, the S/S of hazardous pollutants using geopolymers are comprehensively summarized in this review. Geopolymer-based S/S of typical cations, including Pb, Zn, Cd, Cs, Cu, Sr, Ni, etc., were involved and elucidated. The S/S mechanisms for cationic heavy metals were concluded, mainly including physical encapsulation, sorption, precipitation, and bonding with a silicate structure. In addition, compared to cationic ions, geopolymers have a poor immobilization ability on anions due to the repulsive effect between them, presenting a high leaching percentage. However, some anions, such as Se or As oxyanions, have been proved to exist in geopolymers through electrostatic interaction, which provides a direction to enhance the geopolymer-based S/S for anions. Besides, few reports about geopolymer-based S/S of organic pollutants have been published. Furthermore, the adsorbents of geopolymer-based composites designed and studied for the removal of hazardous pollutants from aqueous conditions are also briefly discussed. On the whole, this review will offer insights into geopolymer-based S/S technology. Furthermore, the challenges to geopolymer-based S/S technology outlined in this work are expected to be of direct relevance to the focus of future research.

Synergistic removal of fluoride from groundwater by seed crystals and bacteria based on microbially induced calcium precipitation

A new hypothesis that seed crystals (SC) and bacteria based on microbially induced calcium precipitation (MICP) synergistically remove fluoride (F^-) from groundwater was proposed, with a focus on evaluating the defluoridation potential of this method and revealing its F^- removal mechanism. The crucial conditions were optimized to reduce preparation and operation costs. SC furnished more available binding sites due to the existence of bacteria, and the reuse experiments showed that the defluoridation efficiency of SC still remained a high level after 14 cycles (70.10%), with a residual F^- concentration of 0.96 mg L^-1. The SEM-EDS, FTIR and XRD analyses indicated the predominant F^- removal mechanism of SC could be ascribed to the chemisorption, ion exchange, and co-precipitation. Moreover, ion exchange and co-precipitation (PO₄^3- involvement) were validated more contributive than chemisorption (CaCO₃ and CaSO₄ involvement). As a feasible, reusable, and eco-friendly technique, SC suggests promising applications in the treatment of fluoride-contaminated groundwater.

Solidification/stabilization of landfill leachate concentrate contaminants using solid alkali-activated geopolymers with a high liquid solid ratio and fixing rate

Landfill leachate concentrate (LLC) is a highly toxic wastewater that contains many refractory contaminants. One of the technical and economic treatment methods is solidification/stabilization (S/S), where the contaminants of LLC can be sealed in one step to achieve zero wastewater discharge. This study presents the S/S of LLC contaminants using solid alkali-activated geopolymers prepared from blast furnace slag (BFS) and powdery sodium silicate. The stability of the formed geopolymer was studied through unconfined compressive strength (UCS) and leaching tests. The strongest UCS was obtained when the modulus of the activator was 1.16 with a high liquid/solid ratio of 0.64. BFS-based geopolymers presented excellent LLC S/S efficiency. The S/S rates of TOC, COD_Cr, NH₃-N, Cl^-, and SO₄^2- were 81%, 89%, 97%, 97%, and 78%, respectively. The S/S rates of heavy metals, i.e., Cd and Pb, were all more than 99%. The results of microstructure characterization showed that the S/S mechanism of LLC pollutants was the dual effect of physical closure and chemical stability. Cl^- and SO₄^2- were respectively stabilized in the crystal lattice by Friedel's salt and calcium sulfate, respectively, while organic matter and NH₃-N were physically encapsulated in the dense structure of the geopolymer. Overall, BFS based geopolymers demonstrated high treatment capacity and excellent S/S efficiency, and have a potential application prospects in LLC treatment.

Batch fluidized bed reactor based modified biosynthetic crystals: Optimization of adsorptive properties and application in fluoride removal from groundwater

A batch fluidized bed reactor (BFBR) with modified biosynthetic crystals (MBC), derived from Pseudomonas sp. HXF1, was investigated for the treatment of the groundwater containing fluoride (F^-). Impacts of different hydraulic retention time (HRT), pH, and initial F^- concentration on F^- removal were examined and the maximum defluorination efficiency was recorded as 95.20%. Moreover, recycling experiments were performed to evaluate the stability of repeated use. BFBR/MBC system showed a long-term effective treatment outcome with low fluctuation in the concentrations of residual Ca²⁺ and F^-. The formed precipitates were characterized by SEM, XPS, XRD, and FTIR. The defluorination mechanisms of BFBR/MBC system were defined as the chemisorption and induced crystallization of Ca₅(PO₄)₃F on the MBC surface. As a feasible, economical, and environment-friendly technique, the method has a long-term value, which suggests promising applications in F^- removal and resourceful treatment.

Efficient photoactivation of peroxymonosulfate by Z-scheme nitrogen-defect-rich NiCo2O4/g-C3N4 for rapid emerging pollutants degradation

Limited peroxymonosulfate (PMS, HSO₄^-) activation efficiency resulted from slow metal reduction has been a challenge in visible-light (vis) assisted sulfate radical-based oxidation. Herein, a Z-scheme photocatalyst composed of nitrogen-defect-rich graphitic carbon nitride nanosheets embedded with nickel cobaltate nanoparticles (NiCo₂O₄/g-C₃N₄-N_vac) was elaborately designed to accelerate Ni(III)/Ni(II) and Co(III)/Co(II) cycles for PMS activation in PMS/vis system. The NiCo₂O₄/g-C₃N₄-N_vac exhibited remarkable enhancement with a tetracycline hydrochloride (TCH) degradation rate constant (0.1168 min^-1), higher than those of NiCo₂O₄/g-C₃N₄ (0.0724 min^-1) and g-C₃N₄ (0.0233 min^-1), respectively. Also, the removal efficiencies of 95.5%, 94.2%, 98.0% and 91.4% for carbamazepine, 4-chlorophenol, atrazine and p-nitrophenol were achieved within 30 min, respectively. Theoretical and experimental results suggested that nitrogen (N) vacancies modulated electric structure to build Z-scheme-charge-transfer platform for rapid reduction of Ni(III) and Co(III), thereby accelerating PMS activation for remarkable removal of emerging pollutants. NiCo₂O₄/g-C₃N₄-N_vac exhibited excellent stability and corresponding electrical energy per order (EE/O) in different water matrix was evaluated. Additionally, TCH degradation behavior, pathways and toxicity of products were analyzed, respectively. This work provided an novel paradigm to design the efficient photo-activator of PMS for environmental remediation.

Facile fabrication of metakaolin/slag-based zeolite microspheres (M/SZMs) geopolymer for the efficient remediation of Cs+ and Sr2+ from aqueous media

Herein we report the fabrication of metakaolin/slag-based geopolymer microspheres by dispersion-suspension-solidification technology, and were then transformed into zeolite microspheres by in-situ thermal curing. The rheological properties and mechanical strength of metakaolin/slag-based zeolite microspheres (M/SZMs) were improved by adding slag. The zeolite microspheres were texturally and morphologically characterized by BET, SEM-EDX and XRD techniques. At 20% slag contents of the total mass of M/SZMs, the specific surface area was significantly increased without changing the structure of the zeolite. Rheological properties analysis of slurry revealed pseudoplastic fluid phase and fitted well to Herschel-Bulkley model. The adsorptive removal data of M/SZMs for Cs⁺ and Sr²⁺ from wastewater followed pseudo-second-order kinetics. The maximum adsorption capacity of M/SZMs for Cs⁺ and Sr²⁺ was 103.74 mg/g and 54.90 mg/g and were best explained by Freundlich and Langmuir isotherm models, respectively. M/SZMs exhibited excellent dynamic separation effect in column-based experimental set up. In addition, M/SZMs also realized outstanding adsorptive removal performance for Cs⁺ and Sr²⁺ from different real wastewater samples. Owing to the simplistic fabrication approach, low cost and highly efficacious nature, M/SZMs can be ranked as alternative candidates for the abatement of Cs⁺ and Sr²⁺ from wastewater.

Isolation of biosynthetic crystals by microbially induced calcium carbonate precipitation and their utilization for fluoride removal from groundwater

Biosynthetic crystals (BC) were prepared through microbially induced calcium carbonate precipitation (MICP) for fluoride (F^-) removal from the groundwater. Batch experiments were conducted to evaluate the fluoride adsorption capacity and the impacts of critical factors (organic matter, pH, initial fluoride concentration and BC dosage) on defluorination efficiency of BC. The maximum adsorption amount and defluorination efficiency were recorded as 5.10 mg g^-1 and 98.24%, respectively. The adsorption kinetics and isotherms studies showed that pseudo-second-order kinetic model and Freundlich isotherm model were best fitting to the reaction. Adsorption thermodynamic parameters indicated a spontaneous, endothermic and thermodynamically favorable adsorption process. Moreover, the mechanism of F^- removal by BC was further analyzed by SEM, XPS, XRD and FTIR. The method can cope with the problem of applying the external organic substances in MICP, and avoid the microbial safety risk in the effluent. As an economically and environmentally friendly adsorbent, BC can be used for F^- removal from groundwater.